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(1) constant definition const

The const in OC can replace macro definition # define.

(1.1) can be read within the constant defined in the program. For example:

Const int a = 3;

Int B = 0;

A = B; // invalid compilation error.

B = a; // valid

(1.2) when using const to define a constant, you must determine that the constant object initializes data.

Const int a = 3; // valid

Const int B; // invalid, resulting in compilation errors

(1.3) when the const constant is referenced in the external link file, after the extern is added, a becomes an object with an external link, and its address has the external link feature.

Extern const int a; // valid

Extern const int B = 3; // The constant cannot be assigned again.

Using const saves more memory space than # define. in the program, const only performs one copy of data, while # define defines the number of objects and the number of data copies.

# Defien a 3; // macro

Const int B = 3; // B is not put into memory at this time

Int I = a; // macro replacement, memory allocation

Int j = B; // memory allocation, which will not be allocated later

Int I = a; // macro replacement, memory allocation again

Int J = B; // no memory is allocated.

(2) array data type definition NSArray

NSArray is an array data class in the Foundation framework. NSArray is like a traditional array in C language. It is usually used in defining integer data types to obtain data storage space in memory. The NSArray object is used to store the subview in the view.

// Create an array variable object and define the "subview" of the scrolling View"

NSArray * subviews = [scrollView1 subviews];